1. Field of the Invention
The present invention relates to a planar single-piece measuring string which is designed to oscillate transversely to the plane defining its rest position and which has two attachment tabs, a resonating string section, two coupling sections connecting the resonating string section to the attachment tabs, and two defined nodal domains where the resonating string section meets the coupling sections.
2. Description of the Related Art
One of the uses of measuring strings of the above-described type is in weighing scales. The measuring string, clamped fast at its ends, is sustained in transverse oscillations whose frequency will vary depending on the weighing load. These frequency variations, processed by appropriate electronic means, serve to determine the force acting on the string at a particular moment.
Previously known from CH-A-648663 is a planar measuring string which is clamped fixedly at both ends of the string. At the transition from the mid-portion, where the actual resonating string section carries out the oscillatory motion, to the coupling sections that adjoin the clamped areas, there are two pairs of flaps oriented at a right angle to the resonating string section and the coupling sections. The four flaps, in order to increase their inertial mass, have supplemental masses attached to them. The two pairs of flaps together with the attached supplemental masses, due to their inertia, form the two nodal points that define the resonating string section therebetween. These nodal masses, being large in relation to the mass of the resonating string section, effectively uncouple the resonating string section from the adjoining parts, i.e., from the clamped portions of the measuring string at both of its ends.
The manufacture of this kind of measuring string is very demanding, considering the extreme accuracy required in producing the stamped sheet-metal parts as well as the supplemental masses that are to be added to the flaps and which, furthermore, have to be joined to the flaps by cementing, soldering or welding. Since the supplemental masses are large in relation to the mass of the resonating string section, this apparatus will work only in a vertical orientation.
From EP-A2-0030741 there is further known an arrangement for isolating the oscillations of a measuring string. This string, again, has two ends configured for its attachment, the actual resonating string section being clamped fixedly between the ends. Disclosed are means, located between the attachment ends and the ends of the clamped resonating string section which oscillates transversely relative to the clamped flat portions, for uncoupling the oscillations caused by the resonating string section. These isolation means include elastic cross-pieces positioned transversally relative to the resonating string section. On the two intermediately positioned transverse cross-pieces which are connected with the ends of the resonating string section, isolating masses are mounted that run parallel to the string. These isolating masses have a resonance frequency that is lower than the frequency of the resonating string section, thereby providing effective isolation between the oscillation of the string and its terminating attachments. The isolating masses, being large in relation to the mass of the resonating string section while having a lower oscillating frequency, prevent the propagation of transverse vibrations into adjacent portions of the apparatus.
From International Patent Application WO 93/10428 there is further known a measuring string which is held clamped at its ends by ring-shaped clamping means. At the mid-point of the resonating string section that extends between the two clamping means, H-shaped weights are attached for the purpose of preventing interference frequencies. In the sections between the supplemental mass at the mid-point of the string and the clamped portions, there are electrical transmitters designed to energize the resonating string section into oscillation. The nodes of the resonating string section are located at the ends of the string near the clamp terminals. In parallel with the resonating string section there may be additional material connections between the clamp terminals for reducing the force acting on the resonating string section. In proportion to their cross-sectional strength they will take up a more or less important share of the tensile force acting between the two clamp terminals and thereby reduce the tensile force acting on the resonating string section. In this known configuration of a measuring string, likewise, there is no positive isolation of the oscillations of the resonating string section from the clamp terminals.
From DE-A1-3709096 there is further known a resonator for the measurement of acceleration of bodies in flight. The resonator includes an oscillating member arranged between two fastening elements, wherein the member oscillates in a plane containing its longitudinal axis and, to isolate it from the fastening elements, is provided at each end with an isolating mass that is connected to the fastening element nearest to it by a pair of isolating springs. Each pair of isolating springs converges or diverges from the isolating mass, wherein the pairs of isolating springs are oriented respectively perpendicular to a line of motion that intersects the location where the isolating spring is connected to the isolating mass, the line of motion representing the direction of the reactive shear force and the reactive moment that exists at the termination of the oscillating member nearest to the respective pair of isolation springs. This known resonator oscillates in the plane that contains the oscillating rod member as well as the isolating masses, the latter being respectively configured in pairs. Uncoupling of the oscillations from the fastening elements is, at best, very difficult to maintain over an extended frequency range.